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INTRODUCTION to STRING FIELD THEORY 1 Warren Siegel 0 0 2 University of Maryland l u College Park, Maryland J 1 1 1 v 4 9 0 7 0 1 0 / h t - p e h : v i X r a Present address: State University of New York, Stony Brook mailto:[email protected] http://insti.physics.sunysb.edu/˜siegel/plan.html CONTENTS Preface 6. Classical mechanics 1. Introduction 6.1. Gauge covariant 120 1.1. Motivation 1 6.2. Conformal gauge 122 1.2. Known models (interacting) 3 6.3. Light cone 125 1.3. Aspects 4 Exercises 127 1.4. Outline 6 7. Light-cone quantum mechanics 2. General light cone 7.1. Bosonic 128 2.1. Actions 8 7.2. Spinning 134 2.2. Conformal algebra 10 7.3. Supersymmetric 137 2.3. Poincaré algebra 13 Exercises 145 2.4. Interactions 16 8. BRST quantum mechanics 2.5. Graphs 19 8.1. IGL(1) 146 2.6. Covariantized light cone 20 8.2. OSp(1,1 2) 157 | Exercises 23 8.3. Lorentz gauge 160 3. General BRST Exercises 170 3.1. Gauge invariance and 9. Graphs constraints 25 9.1. External fields 171 3.2. IGL(1) 29 9.2. Trees 177 3.3. OSp(1,1 2) 35 9.3. Loops 190 | 3.4. From the light cone 38 Exercises 196 3.5. Fermions 45 10. Light-cone field theory 197 3.6. More dimensions 46 Exercises 203 Exercises 51 11. BRST field theory 4. General gauge theories 11.1. Closed strings 204 4.1. OSp(1,1 2) 52 11.2. Components 207 | 4.2. IGL(1) 62 Exercises 214 4.3. Extra modes 67 12. Gauge-invariant interactions 4.4. Gauge fixing 68 12.1. Introduction 215 4.5. Fermions 75 12.2. Midpoint interaction 217 Exercises 79 Exercises 228 5. Particle References 230 5.1. Bosonic 81 Index 241 5.2. BRST 84 5.3. Spinning 86 5.4. Supersymmetric 95 5.5. SuperBRST 110 Exercises 118 PREFACE First, I’d like to explain the title of this book. I always hated books whose titles began “Introduction to...” In particular, when I was a grad student, books titled “Introduction to Quantum Field Theory” were the most difficult and advanced text- books available, and I always feared what a quantum field theory book which was not introductory would look like. There is now a standard reference on relativistic string theory by Green, Schwarz, and Witten, Superstring Theory [0.1], which con- sists of two volumes, is over 1,000 pages long, and yet admits to having some major omissions. Now that I see, from an author’s point of view, how much effort is necessary to produce a non-introductory text, the words “Introduction to” take a more tranquilizing character. (I have worked on a one-volume, non-introductory text on another topic, but that was in association with three coauthors.) Furthermore, these words leave me the option of omitting topics which I don’t understand, or at least being more heuristic in the areas which I haven’t studied in detail yet. The rest of the title is “String Field Theory.” This is the newest approach to string theory, although the older approaches are continuously developing new twists and improvements. The main alternative approach is the quantum mechanical (/analog-model/path-integral/interacting-string-picture/Polyakov/conformal- “field- theory”) one, which necessarily treats a fixed number of fields, corresponding to homogeneous equations in the field theory. (For example, there is no analog in the mechanics approach of even the nonabelian gauge transformation of the field theory, which includes such fundamental concepts as general coordinate invariance.) It is also an S-matrix approach, and can thus calculate only quantities which are gauge-fixed (although limited background-field techniques allow the calculation of 1-loop effective actions with only some coefficients gauge-dependent). In the old S-matrix approach to field theory, the basic idea was to start with the S-matrix, and then analytically continue toobtainquantities whichareoff-shell(andperhapsinmoregeneralgauges). However, in the long run, it turned out to be more practical to work directly with field theory Lagrangians, even for semiclassical results such as spontaneous symmetry breaking and instantons, which change the meaning of “on-shell” by redefining the vacuum to be a state which is not as obvious from looking at the unphysical-vacuum S-matrix. Of course, S-matrix methods are always valuable for perturbation theory, but even in perturbation theory it is far more convenient to start with the field theory in order to determine which vacuum to perturb about, which gauges to use, and what power-counting rules can be used to determine divergence structure without specific S-matrix calculations. (More details on this comparison are in the Introduction.) Unfortunately, string field theory is in a rather primitive state right now, and not even close to being as well understood as ordinary (particle) field theory. Of course, this is exactly the reason why the present is the best time to do research in this area. (Anyonewhocanhonestlysay, “I’lllearnitwhenit’sbetterunderstood,” shouldmark a dateon hiscalendar for returning to graduateschool.) It istherefore simultaneously the best time for someone to read a book on the topic and the worst time for someone to write one. I have tried to compensate for this problem somewhat by expanding on the more introductory parts of the topic. Several of the early chapters are actually on the topic of general (particle/string) field theory, but explained from a new point of view resulting from insights gained from string field theory. (A more standard course on quantum field theory is assumed as a prerequisite.) This includes the use of a universal method for treating free field theories, which allows the derivation of a single, simple, free, local, Poincaré-invariant, gauge-invariant action that can be applied directly to any field. (Previously, only some special cases had been treated, and each in a different way.) As a result, even though the fact that I have tried to make thisbookself-contained withregardtostring theoryingeneralmeans thatthere is significant overlap with other treatments, within this overlap the approaches are sometimesquitedifferent, andperhapsinsomewayscomplementary. (Thetreatments of ref. [0.2] are also quite different, but for quite different reasons.) Exercises are given at the end of each chapter (except the introduction) to guide the reader to examples which illustrate the ideas in the chapter, and to encourage him to perform calculations which have been omitted to avoid making the length of this book diverge. This work was done at the University of Maryland, with partial support from the National Science Foundation. It is partly based on courses I gave in the falls of 1985 and 1986. I received valuable comments from Aleksandar Miković, Christian Preitschopf, Anton van de Ven, and Harold Mark Weiser. I especially thank Barton Zwiebach, who collaborated with me on most of the work on which this book was based. June 16, 1988 Warren Siegel Originally published 1988 by World Scientific Publishing Co Pte Ltd. ISBN 9971-50-731-5, 9971-50-731-3 (pbk) July 11, 2001: liberated, corrected, bookmarks added (to pdf) 1.1. Motivation 1 1. INTRODUCTION 1.1. Motivation The experiments which gave us quantum theory and general relativity are now quite old, but a satisfactory theory which is consistent with both of them has yet to be found. Although the importance of such a theory is undeniable, the urgency of finding it may not be so obvious, since the quantum effects of gravity are not yet accessible to experiment. However, recent progress in the problem has indicated that the restrictions imposed by quantum mechanics on a field theory of gravitation are so stringent as to require that it also be a unified theory of all interactions, and thus quantum gravity would lead to predictions for other interactions which can be subjected to present-day experiment. Such indications were given by supergravity theories [1.1], where finiteness was found at some higher-order loops as a consequence of supersymmetry, which requires the presence of matter fields whose quantum effects canceltheultraviolet divergences ofthegravitonfield. Thus, quantumconsistency led tohighersymmetrywhichinturnledtounification. However, eventhissymmetrywas foundinsufficient to guarantee finiteness at allloops [1.2](unless perhaps the graviton were found to be a bound-state of a truly finite theory). Interest then returned to theories which had already presented the possibility of consistent quantum gravity theories as a consequence of even larger (hidden) symmetries: theories of relativistic strings [1.3-5]. Strings thus offer a possibility of consistently describing all of nature. However, even if strings eventually turn out to disagree with nature, or to be too intractable to be useful for phenomenological applications, they are still the only consistent toy models of quantum gravity (especially for the theory of the graviton as a bound state), so their study will still be useful for discovering new properties of quantum gravity. Thefundamentaldifferencebetween aparticleandastringisthataparticleisa0- dimensional object in space, with a 1-dimensional world-line describing its trajectory in spacetime, while a string is a (finite, open or closed) 1-dimensional object in space, which sweeps out a 2-dimensional world-sheet as it propagates through spacetime: 2 1. INTRODUCTION ## # # x x(τ) # # # # particle c c r c c c c cc ## # # # X(σ) X(σ,τ) # # ## # # # # # # # c string c c c c c cc c c c c c c cc The nontrivial topology of the coordinates describes interactions. A string can be either open or closed, depending on whether it has 2 free ends (its boundary) or is a continuous ring (no boundary), respectively. The corresponding spacetime figure is then either a sheet or a tube (and their combinations, and topologically more complicated structures, when they interact). Strings were originally intended to describe hadrons directly, since the observed spectrum and high-energy behavior of hadrons (linearly rising Regge trajectories, which in a perturbative framework implies the property of hadronic duality) seems realizable only in a string framework. After a quark structure for hadrons became generally accepted, it was shown that confinement would naturally lead to a string formulation of hadrons, since the topological expansion which follows from using 1/N as a perturbation parameter (the only dimensionless one in massless QCD, color besides1/N ), aftersummationintheotherparameter(thegluoncoupling, which flavor becomes the hadronic mass scale after dimensional transmutation), is the same per- 1.2. Known models (interacting) 3 turbation expansion as occurs in theories of fundamental strings [1.6]. Certain string theories can thus be considered alternative and equivalent formulations of QCD, just as general field theories can be equivalently formulated either in terms of “fundamental” particles or in terms of the particles which arise as bound states. However, in practice certain criteria, in particular renormalizability, can be simply formulated only in one formalism: For example, QCD is easier to use than a theory where gluons are treated as bound states of self-interacting quarks, the latter being a nonrenormalizable theory which needs an unwieldy criterion (“asymptotic safety” [1.7]) to restrict the available infinite number of couplings to a finite subset. On the other hand, atomic physics is easier to use as a theory of electrons, nuclei, and photons than a formulation in terms of fields describing self-interacting atoms whose exci- tations lie on Regge trajectories (particularly since QED is not confining). Thus, the choice of formulation is dependent on the dynamics of the particular theory, and perhaps even on the region in momentum space for that particular application: perhaps quarks for large transverse momenta and strings for small. In particular, the running of the gluon coupling may lead to nonrenormalizability problems for small transverse momenta [1.8] (where an infinite number of arbitrary couplings may show up as nonperturbative vacuum values of operators of arbitrarily high dimension), and thus QCD may be best considered as an effective theory at large transverse momenta (in the same way as a perturbatively nonrenormalizable theory at low energies, like the Fermi theory of weak interactions, unless asymptotic safety is applied). Hence, a string formulation, where mesons are the fundamental fields (and baryons appear as skyrmeon-type solitons [1.9]) may be unavoidable. Thus, strings may be important for hadronic physics as well as for gravity and unified theories; however, the presently known string models seem to apply only to the latter, since they contain massless particles and have (maximum) spacetime dimension D = 10 (whereas confinement in QCD occurs for D 4). ≤ 1.2. Known models (interacting) Although many string theories have been invented which are consistent at the tree level, most have problems at the one-loop level. (There are also theories which are already so complicated at the free level that the interacting theories have been too difficult to formulate to test at the one-loop level, and these will not be discussed here.) These one-loop problems generally show up as anomalies. It turns out that the anomaly-free theories are exactly the ones which are finite. Generally, topologi- 4 1. INTRODUCTION cal arguments based on reparametrization invariance (the “stretchiness” of the string world sheet) show that any multiloop string graph can be represented as a tree graph withmany one-loopinsertions [1.10],so alldivergences shouldberepresentable asjust one-loop divergences. The fact that one-loop divergences should generate overlapping divergences then implies that one-loop divergences cause anomalies in reparametrization invariance, since the resultant multi-loop divergences are in conflict with the one-loop-insertion structure implied by the invariance. Therefore, finiteness should be a necessary requirement for string theories (even purely bosonic ones) in order to avoid anomalies in reparametrization invariance. Furthermore, the absence of anomalies in such global transformations determines the dimension of spacetime, which in all known nonanomalous theories is D = 10. (This is also known as the “critical,” or maximum, dimension, since some of the dimensions can be compactified or otherwise made unobservable, although the number of degrees of freedom is unchanged.) In fact, there are only four such theories: I: N=1 supersymmetry, SO(32) gauge group, open [1.11] IIA,B: N=2 nonchiral or chiral supersymmetry [1.12] heterotic: N=1 supersymmetry, SO(32) or E E [1.13] 8 8 ⊗ or broken N=1 supersymmetry, SO(16) SO(16) [1.14] ⊗ All except the first describe only closed strings; the first describes open strings, which produce closed strings as bound states. (There are also many cases of each of these theories due to the various possibilities for compactification of the extra dimensions onto tori or other manifolds, including some which have tachyons.) However, for sim- plicitywewillfirstconsidercertaininconsistent theories: thebosonicstring,whichhas global reparametrization anomalies unless D = 26 (and for which the local anomalies described above even for D = 26 have not yet been explicitly derived), and the spinning string, which is nonanomalous only when it is truncated to the above strings. The heterotic strings are actually closed strings for which modes propagating in the clockwise direction are nonsupersymmetric and 26-dimensional, while the counter- clockwise ones are N = 1 (perhaps-broken) supersymmetric and 10-dimensional, or vice versa. 1.3. Aspects There are several aspects of, or approaches to, string theory which can best be classified by the spacetime dimension in which they work: D = 2,4,6,10. The 2D 1.3. Aspects 5 approach is themethod of first-quantization in the two-dimensional world sheet swept out by the string as it propagates, and is applicable solely to (second-quantized) perturbation theory, for which it is the only tractable method of calculation. Since it discusses only the properties of individual graphs, it can’t discuss properties which involve an unfixed number of string fields: gauge transformations, spontaneous symmetry breaking, semiclassical solutions to the string field equations, etc. Also, it can describe only the gauge-fixed theory, and only in a limited set of gauges. (However, by introducing external particle fields, a limited amount of information on the gauge- invariant theory can be obtained.) Recently most of the effort in this area has been concentrated on applying this approach to higher loops. However, in particle field theory, particularly for Yang-Mills, gravity, and supersymmetric theories (all of which are contained in various string theories), significant (and sometimes indispensable) improvements in higher-loop calculations have required techniques using the gauge- invariant field theory action. Since such techniques, whose string versions have not yet been derived, could drastically affect the S-matrix techniques of the 2D approach, we do not give the most recent details of the 2D approach here, but some of the basic ideas, and the ones we suspect most likely to survive future reformulations, will be described in chapters 6-9. The 4D approach is concerned with the phenomenological applications of the low-energy effective theories obtained from the string theory. Since these theories are still very tentative (and still too ambiguous for many applications), they will not be discussed here. (See [1.15,0.1].) The 6D approach describes the compactifications (or equivalent eliminations) of the 6 additional dimensions which must shrink from sight in order to obtain the observed dimensionality of the macroscopic world. Unfortunately, this approach has severalproblemswhichinhibitausefultreatmentinabook: (1)Sofar,nojustification has been given as to why the compactification occurs to the desired models, or to 4 dimensions, or at all; (2) the style of compactification (Ka luz˙a-Klein, Calabi-Yau, toroidal, orbifold, fermionization, etc.) deemed most promising changes from year to year; and (3) the string model chosen to compactify (see previous section) also changes every few years. Therefore, the 6D approach won’t be discussed here, either (see [1.16,0.1]). What is discussed here is primarily the 10D approach, or second quantization, which seeks to obtain a more systematic understanding of string theory that would allow treatment of nonperturbative as well as perturbative aspects, and describe the 6 1. INTRODUCTION enlargedhidden gaugesymmetries which give string theoriestheir finiteness andother unusual properties. In particular, it would be desirable to have a formalism in which allthesymmetries(gauge,Lorentz,spacetimesupersymmetry) aremanifest,finiteness follows from simple power-counting rules, and all possible models (including possible 4D models whose existence is implied by the 1/N expansion of QCD and hadronic duality) can be straightforwardly classified. In ordinary (particle) supersymmetric field theories [1.17], such a formalism (superfields or superspace) has resulted in much simpler rules for constructing general actions, calculating quantum corrections (su- pergraphs), and explaining all finiteness properties (independent from, but verified by, explicit supergraph calculations). The finiteness results make use of the background field gauge, which can be defined only in a field theory formulation where all symmetries are manifest, and in this gauge divergence cancellations are automatic, requiring no explicit evaluation of integrals. 1.4. Outline String theory can be considered a particular kind of particle theory, in that its modes of excitation correspond to different particles. All these particles, which differ in spin and other quantum numbers, are related by a symmetry which reflects the properties of the string. As discussed above, quantum field theory is the most com- plete framework within which to study the properties of particles. Not only is this framework not yet well understood for strings, but the study of string field theory has brought attention to aspects which are not well understood even for general types of particles. (This is another respect in which the study of strings resembles the study of supersymmetry.) We therefore devote chapts. 2-4 to a general study of field theory. Rather than trying to describe strings in the language of old quantum field theory, we recast the formalism of field theory in a mold prescribed by techniques learned from the study of strings. This language clarifies the relationship between physical states and gauge degrees of freedom, as well as giving a general and straightforward method for writing free actions for arbitrary theories. In chapts. 5-6 we discuss the mechanics of the particle and string. As mentioned above, this approach is a useful calculational tool for evaluating graphs in perturbation theory, including the interaction vertices themselves. The quantum mechanics of the string is developed in chapts. 7-8, but it is primarily discussed directly as an operator algebra for the field theory, although it follows from quantization of the classical mechanics of the previous chapter, and vice versa. In general, the procedure of